Three phases generator

Dear all,
please could you share with me your knowledge about three phases generator? I would like to know exactly how current flow back to generator. For example what is exact way of current from generator (1). What line is using as return path for it. And also do not know how it is possible that phasors I1 + I2 + I3 = 0.

In the absence of a return path (which is how a delta arrangement works) any impedance will result in a variation in the voltages in the three phases - the 'ground' will shift away from 0V. It's a bit harder to picture than the wye system.